bug#39295: 26.3; kbd-help property is undocumented

[Eli Zaretskii]

> From: Howard Melman <hmelman@gmail.com>
> Date: Fri, 31 Jan 2020 11:10:26 -0500

[Please keep the bug address on the CC list.]

> > It indeed is unused, so I'm not sure it should be documented at this
> > time.  As for "new": the help-at-pt.el file was added to Emacs 17
> > years ago, and its original version already included support for this
> > property.
> 
> Perhaps it should be deprecated? Without documentation it's unlikely to be 
> used.

It is documented in the doc strings, isn't it?

> It would be nice if some place described it as unused (and perhaps as unused 
> for a very long time). 

Unused in Emacs itself doesn't mean no one out there uses it.  A
feature that is unused doesn't do any harm.

> Since the code will use it, it should remain in the docstring. If you don't 
> want to add it to the manual then may I suggest at a least a comment in the 
> code saying so and what its intent was, perhaps in the package commentary, 
> would be useful.

Not every feature is documented in the manual, only the important ones
are.  And I'm not sure I see the utility of stating in comments that
we don't document this in the manual: we don't say that about all the
other features not mentioned in the manual.